This proposal, entitled,"Mechanisms of Leptin Receptor Signal Attenuation in vivo, "is an application for renewal of DK57768. The long (or LRb) isoform of the leptin receptor mediates signaling and the physiologic action of leptin to regulate energy balance (decreasing feeding and increasing energy expenditure) and neuroendocrine function. It is not clear why leptin fails to adequately protect from obesity and the predisposition to Type 2 diabetes in humans; it is thus critical to understand the molecular details of LRb signaling and especially mechanisms by which LRb signaling is attenuated in order to understand potential mechanisms of leptin resistance and/or to identify potential targets for the therapy of obesity. The long-term outlook of our previous and future studies is to understand the regulation and mechanisms of signaling by LRb and their role in physiologic leptin action. LRb is a type 1 cytokine receptor that, although devoid of enzymatic activity, mediates phosphotyrosine-dependent signaling by means of an associated Jak2 tyrosine kinase and two phosphorylation sites on the intracellular LRb. Tyr1138 activates STAT3, which is responsible for important positive leptin signals as well as for the induction of SOCS3 by leptin. In addition to its role in SHP- 2/ERK signaling in cultured cells, Tyr985 binds SOCS3 to mediate some elements of LRb signal attenuation in vivo as well as in cultured cells; SOCS3 also binds to the LRb-associated Jak2 to directly inhibit Jak2 signaling. We propose to study the function of SOCS3 binding to Jak2 and Tyr985 in signal attenuation in cultured cells and in vivo. In addition to shedding light upon the biology of LRb specifically, our analysis will illuminate basic mechanisms of cytokine receptor/tyrosine kinase signaling. We propose: Specific Aim 1: Determine the molecular mechanisms of LRb signal attenuation in cultured cells. Specific Aim 2: Examine the role of LRb phosphorylation sites in leptin sensitivity in vivo. Specific Aim 3: Address the mechanism(s) of SOCS3-mediated inhibition of LRb action in vivo. Each of these aims is crucial to addressing the central hypothesis of this proposal- that leptin-induced SOCS3 expression mediates feedback inhibition on LRb action in vi\o by multiple mechanisms, and that each of these mechanisms differentially affects leptin sensitivity in mammals.